Countertop machines of this type vary from hand cranked units used to make homemade ice cream to high capacity freezers used by restaurants and bars. In the latter, a cooling cylinder is typically held in a tank for the product, and ice crystals formed on the heat-conductive outer wall of the cooling cylinder are scraped off by the auger. The cooling cylinder is the evaporator of a vapour-compression refrigeration circuit which typically further includes a compressor, a condenser and a restriction device. The restriction device controls refrigerant flow and releases the pressure between the high-pressure condensation side and the low-pressure evaporation side and is most commonly a capillary tube.
Domestic appliances for producing frozen food products, unlike the commercially used versions, are not maintained at operating temperature and so, typically, for each batch it is necessary to reduce the temperature of not just the product to be frozen, but also of the cooling cylinder, from ambient temperature. It will therefore be understood that there is a need to achieve a short freezing time, following start-up, with a small refrigeration capacity. Moreover, manufacturers also must consider the need to most cost-effectively produce a range of models, such as a small-footprint model suitable to making two or three servings at one time, to larger capacity machines suitable for a family.
Perhaps the most common cooling cylinder has a cylindrical wall scraped by the auger and on the inside of which is a helical tube coil carrying the refrigerant. Following start-up, the large liquid component of the refrigerant rapidly evaporates on meeting the room-temperature evaporator coil, producing a complex two-phase flow through the coil. As more refrigerant is introduced, the liquid component is distributed throughout the greatest part of the length of the coil, as the vapour flow is able to push along slugs of liquid, as well as pass through an annular film of liquid refrigerant wetting the walls of the coils. Since most heat transfer occurs when the liquid refrigerant is changed to vapour, there is a drop in heat transfer toward the outlet of the coil, but the wide distribution of the liquid component along the coil tends to fairly evenly distribute the heat transfer. On the one hand, even distribution of the heat transfer increases efficiency when operating at capacity, but may reduce efficiency when a small batch, such as a single serving is made.
It is an object of the present invention to overcome or substantially ameliorate the above disadvantages or, more generally, to provide an improved auger-type ice maker appliance.